This invention relates to an electroluminescent (EL) device for use in matrix type EL display systems or the exposing unit electronic imaging systems. More particularly, this invention relates to an EL device that permits the use of amorphous silicon (a-Si) in the semiconductor layer of a thin-film transistor (TFT) that drives an EL cell.
An equivalent circuit for a conventional EL device for one bit of pixel in a matrix type EL display system or an array of EL cells is shown in FIG. 6. The EL device is composed of a first switching element Q.sub.1 (TFT), a storage capacitor Cs connected at one terminal to the source terminal of the switching element Q.sub.1, a second switching element Q.sub.2 (TFT) whose gate terminal is connected to the source terminal of the first switching element Q.sub.1 and whose source terminal is connected to the other terminal of the storage capacitor Cs, and an EL cell C.sub.EL connected at one terminal to the drain terminal of the second switching element Q.sub.2 and at the other terminal to an EL driving power source Va. The first switching element Q.sub.1 turns on in response to a switching signal SCAN applied to its gate terminal and as this first switching element Q.sub.1 turns on or off, data is written into the storage capacitor Cs in response to a luminescence signal DATA. Namely, when a luminescence signal DATA (H) is written into the storage capacitor Cs, the resulting voltage is applied to the gate terminal of the second switching element Q.sub.2 to turn it on, causing the EL cell C.sub.EL to emit light with power supplied from the drive source Va. When the luminescence signal DATA is at low (L) level, the storage capacitor Cs is discharged via the first switching element Q.sub.1.
When the second switching element Q.sub.2 is off, the power from the drive source Va is applied between its drain and source electrodes. Therefore, the second switching element Q.sub.2 is required to withstand voltage about twice as high as is supplied from the drive source Va. The element is also required to have a correspondingly low current characteristic. To meet these conditions, the semiconductor layer of the switching element has been made from limited materials exemplified by cadmium selenite (CdSe) and polycrystalline silicon (poly-Si). However, cadmium selenite (CdSe) has had the problem that its drain voltage vs. drain current characteristic is unstable and varies with time to present difficulty in maintaining a constant level of luminance of the EL element C.sub.EL. As for poly-Si, a high process temperature is necessary to form a poly-Si layer and this makes poly-Si unsuitable for the purpose of fabricating a large-area device by integrating the EL cell C.sub.EL and the switching element Q.sub.2 into a unitary assembly on a common substrate.
With a view to solving those problems associated with cadmium selenite (CdSe) and polycrystalline silicon (poly-Si), a switching element having the configuration shown in FIG. 7 has been proposed. This switching element uses amorphous silicon (a-Si) for a semiconductor layer 4 and to increase the withstand voltage of the second switching element Q.sub.2, its gate electrode 2' is offset in position towards the source electrode 6b. As FIG. 3 shows, the drain voltage vs. drain current characteristic of this switching element is satisfactory in terms of the off-state withstand voltage and drains current. However, if the drain voltage is of negative polarity, only a small drain current will flow. More specifically, if the drain voltage is of negative polarity with respect to the ac signal necessary to drive the EL cell C.sub.EL, no adequate drain current can be insured as indicated by dashed line A in FIG. 3. To cope with this situation, the drive voltage has to be increased in order to enhance the luminance of the EL cell C.sub.EL but then increased power loss will occur. In addition, the switching element Q.sub.2 is required to withstand an even higher voltage.